The use of AC induction motors has become commonplace. Many ordinary appliances and much of the equipment used in residential as well as in industrial and commercial settings utilize such motors. The motors are ordinarily connected to power lines provided by local utility companies, which can vary substantially in voltage between locales and over time. Induction motors typically operate at relatively constant speeds, the speed being independent of the applied AC voltage to the motor over a range of operating voltages.
Unfortunately, induction motors utilize significant power when operating without a load. Specifically, the current drawn by the motor is generally constant, and depends on the voltage applied to the motor. Therefore, it would be desirable to decrease the voltage to the motor when the motor is not loaded, thereby decreasing energy used by the motor. However, most motors are operated by line voltages that are not adjustable by the user. Even where voltage may be adjusted, it is difficult to make the necessary adjustments to quickly respond to changes in load.
The energy consumption of an induction motor is determined from the integral over a predetermined period of the product of the instantaneous AC voltage applied across the motor terminals and the instantaneous AC current through the motor. Typical AC line voltages are sinusoidal. It is known that applying a sinusoidal input to an induction motor will result in both the AC voltage and AC current having the same sine wave shape but offset in time. The time offset between voltage and current is called a phase shift or phase difference and is typically expressed as an angle. For a constant voltage and, hence, relatively constant current, the power consumed by an induction motor may be expressed as Vlcosc.phi., where V is the average value of the applied AC voltage across the motor, I is the average value of the AC current through the motor and .phi. is the phase difference between the voltage and the current. Cos .phi. is sometimes referred to as the "power factor". Thus, power consumption is related to the phase difference between the AC voltage applied to the motor and the AC current through the motor. It is well known that the phase difference between the voltage and current in an induction motor and, therefore, power consumption changes with changes in the load applied to the motor. However, when the motor is unloaded the power factor remains large enough to result in substantial wasted energy due to the relatively large current which flows through the motor. While it may, in theory, be possible to maximize the efficiency of an induction motor which is subject to a constant load, many, if not most, applications for such motors involve loads which vary over time.
One method for reducing the energy consumption of an induction motor utilized in the prior art will be referred to as Power Factor Control or PFC. By measuring changes in the phase difference between the voltage and current, changes in power consumption and, thus, in applied load may be detected. This prior art method of PFC involves measuring the phase difference between the voltage and current and using this measured information to interrupt the application of line voltage to the motor for a portion of each AC cycle. By varying the duration of the interruptions of the AC voltage in response to changes in the phase difference between the current and the voltage it is possible to adjust the rms (root mean square) value of the applied voltage. Thus, when the motor is lightly loaded, i.e., .phi. is large, the rms voltage is reduced. On the other hand, when the motor is fully loaded, i.e., .phi. is small, the rms voltage is increased, i.e., the interruptions of the line voltage applied to the motor are minimized or eliminated.
An example of an apparatus utilizing this PFC approach is disclosed by Nola in U.S. Pat. No. 4,052,648. Nola teaches measuring the phase difference between current and voltage and using the measured information to control the duration of the voltage to an induction motor by means of a triac. A triac is a well known device controlled by a gate which can act to interrupt voltage applied to the motor. Nola measures the voltage applied across the motor by means of a center tap transformer whose primary coil is connected in parallel with the motor. The center tap transformer produces two oppositely phased voltage signals from the terminals of its secondary. These two voltages signals are then passed through a square wave shaper, which is at a uniform high value when AC voltage is positive and is uniformly low when AC voltage is negative. This shaping removes all amplitude information while maintaining polarity information.
Simultaneously, the current is detected by a second transformer, the output of which is also passed through a square wave shaper. The square wave output is then differentiated, creating a series of spikes which indicate moments when the current switches direction and is therefore at zero. These points are referred to as zero point crossings. These spikes are ted into a one-shot circuit, which generates a square wave output. Next, the voltage square wave and the current square wave are multiplied. The resulting rectangular wave consists of pulses with a width related to the phase difference between the current and voltage squarewaves. This signal is then integrated, and the output is monitored. If the load decreases, the phase angle between the current and voltage changes, and the pulse width then changes. Such changes cause the gate control circuit to disengage the triac for a longer portion of each AC cycle, decreasing the rms voltage applied to the motor and energy consumption.
It is believed that the apparatus described has not performed well in practice and has not been commercially successful. The probable explanation for these problems is the complexity of the apparatus. While numerous attempts have been made to diminish this complexity, no prior art Power Factor Control system known to the inventor has overcome these problems.
An example of an attempt to overcome the complexity of the apparatus described in the '648 patent is set forth by Nola in U.S. Pat. No. 4,266,177. In this second patent, Nola teaches a system which also relies on monitoring the phase difference between the voltage and current using different circuitry. Nola's second approach includes generating first and second square wave signals from the AC operating voltage across the motor leads and from the current passing through the motor, respectively. These square wave signals are then summed and integrated to generate a signal which is transmitted to the non-inverting input of an operational amplifier. The edges of these signals are also detected and are used to time a ramp generator. The output of the ramp generator is transmitted to the inverting input of that same operational amplifier. The output of the operational amplifier is the difference between the average value of the summed signal and the value from the ramp generator. The phase difference between current and voltage is measured by the width of the summed signal. Wider pulses yield larger integrated outputs, which are then transmitted to the operational amplifier. Therefore, an increase in the phase difference will result in a larger difference signal from the operational amplifier. This difference signal is used to control a triac which controls AC voltage to the motor.
This apparatus continues to rely upon removing magnitude information from the detected voltage and current signals, and requires complex circuitry to accomplish control of the applied motor voltage. Again, it is believed that the apparatus described in the '177 patent has not enjoyed commercial success.
Most induction motors are designed to operate adequately at predetermined line voltages. Normally, the motor designer must assume that the motor will be operated at the lowest line voltage normally encountered. Such a voltage may be far lower that the normal line voltage available at most locations and at most times. For example, a motor used in a refrigerator must be capable of delivering adequate power under full load during a "brown-out" condition, i.e., when a utility reduces line voltage over its entire grid (or portion thereof) in response to unusually high electrical energy demand. Changes in line voltage affect both motor performance and energy consumption. Wide variations in line voltage are undesirable. Unfortunately, such variations are beyond the control of most motor designers and users. It is noted that ordinary line voltage fluctuations will not result in changes in the phase between the current and the voltage. Therefore, prior art PFC systems will not respond to fluctuations in line voltage.
Therefore, there is a need for a energy savings system for controlling the voltage applied to an induction motor which is simple, which is responsive to changes in line voltage to adjust for such changes, and which is responsive to changes in motor loading to adjust for such changes.
Accordingly, it is an object of the present invention to provide an improved induction motor control system for energy savings.
Another object of the invention is to provide an energy savings system for use with induction motors which are simpler in design than the prior art.
These and other objects of the invention will become apparent to those skilled in the art from the following description and accompanying claims and drawings.